The benefits of fire-resistant doors (sometimes referred to hereinafter as "fire doors") have long been recognized. For safety purposes, these are frequently required by building codes, especially for commercial and institutional structures. For example, fire doors are frequently specified for interior doors of hotels, hospitals, schools, and so forth.
In the past, such fire doors have ordinarily been constructed of metal. This material has been satisfactory for commercial and institutional installations, but there has been an increasing movement to specify fire doors for residential construction, in order to translate their benefits into the home. For example, it is becoming common to require installation of a fire door between the interior of a house and an attached garage, because of the likelihood of a fire arising in the latter. However, the metal construction of traditional fire doors, particularly the metal door skins, is generally considered unacceptable for home use from the standpoint of aesthetics, being that wood-skinned doors are greatly preferred by residential customers.
There have been attempts to provide wood-skinned fire doors, but the requisite resistance to burn-through has generally been achieved at the expense of costly construction and poor thermal insulation qualities. For example, some doors have been built around an expensive solid slab of fire-resistant mineral material which extends the full thickness of the door, and which possesses poor insulation qualities. Other approaches have generally had the disadvantages of complex and expensive construction, and sometimes severely compromised fire-resistant qualities. An example of this is the door construction shown in U.S. Pat. No. 4,282,687 (Teleskivi). In this construction, there is a door having an outer frame made up of edge timbers with a subframe being constructed within this. The subframe is built up from a number of metal members having a Z-shaped cross section. These crisscross both faces of a central panel made of glass fiber reinforced gypsum sheet, and are held together by screws 16 which pass through this. The outer flanges of the Z-shaped members support a door skin made of fiberboard and wood veneer, and layers of mineral fiber compressed bats 22 are positioned between these skins and the central core.
In short, while the approach which has been taken by Teleskivi may represent an advance in some respects, the results are far from satisfactory in practice. Firstly, constructing the subframes from numerous metal members represents a laborious and costly process. Also, the metal frameworks lying on opposite sides of the gypsum core are connected through this by the metal securing screws, and these represent an uninterrupted conduction path for the transmission of thermal energy; it would therefore appear likely that a fire on one side of the door would quickly destroy the veneer and pressboard skin to reach the first metal framework, and then the heat would be conducted via this and the connecting screws to the opposite framework, setting the opposite skin ablaze. As for the mineral fiber bats, the thermal insulation offered by these would appear to be marginal at best. Moreover, simply from the standpoint of long-term durability, the structure appears to rely largely on the built-up metal framework, and this would be susceptible to flexing and working loose over many years of use, especially where the metal strips engage the outer framework of the door.
Accordingly, a need exists for an energy-efficient fire door having a construction such that this is suitable for use as an exterior door in a residential structure, which exhibits superior fire resistant qualities, and which has a high insulation factor so as to minimize energy waste due to heat loss. Furthermore, there exists a need for such a door which is simple and inexpensive to construct, which is very durable in order to meet the requirements of long-term residential use, and which also presents an aesthetic aspect which is suitable for such applications.